PROJECT SUMMARY Persistent colonization is a key facet of microbe-host interactions. Little is known about the mechanisms that allow a bacterium to persistently colonize its host. We have developed a mouse model for dissecting bacterial and host determinants of persistent colonization. It pairs a commensal of wild mice, Neisseria musculi, with the lab mouse, which is not naturally colonized with Neisseria. N. musculi is closely related to commensal and pathogenic species of Neisseria, and encode many host interaction factors and vaccine candidates of these pathogens. A single oral dose of N. musculi results in persistent, asymptomatic colonization of the oral cavity and gut of the mouse for at least 1 year. Colonization requires not only the Type IV pilus (Tfp) fiber but also retraction of the fiber. ?pilE, which does not express the pilus fiber subunit and ?pilT, which does not express the pilus retraction motor, both fail to colonize mice. Here, we will use our newly developed model to examine the role of Tfp retraction on N. musculi colonization and persistence. We will also use spectral imaging Fluorescence In Situ Hybridization (FISH) to locate and characterize the biofilms formed by N. musculi and its Tfp retraction mutant in the mouse alimentary tract.